When Takeshi Ishida was drafted into the Japanese military in 1943, he believed that he was fighting a just war to liberate Asia from western colonialism. It was only after taking part in daily exercises in which he was trained to kill that Ishida began to question Japanese militarism.

When the full horror of Japanese atrocities became apparent in the aftermath of its surrender, Ishida devoted himself to defending the pacifist constitution, imposed by victorious US occupation authorities. Now, the former imperial army officer is consumed with fear that young Japanese will again be sent to fight overseas, following the most dramatic shift in the country's defence policy for almost 70 years.

On Tuesday, the conservative prime minister, Shinzo Abe, and his cabinet agreed to lift the longstanding ban on Japan's troops engaging in combat overseas, a move Ishida believes could once again drag his county into a reckless war.

"What Abe is doing is destroying the principles of our pacifist constitution," Ishida, a professor emeritus at Tokyo University, told the Guardian. "Not killing anyone abroad is, in a sense, a precious part of our heritage. Why should we have to throw it away on the orders of one man rather than through the will of the people?"

Ishida, 91, believes Japan's US-authored constitution is at the heart of its postwar peace. "I came to realise that I had been indoctrinated to become a militaristic youth," he said. "As someone who felt for a while that he had lost his identity, the peace constitution was a great encouragement."

That tradition of pacifism is in danger of being abandoned as Abe moves to reinterpret the constitution and lift the self-imposed ban on collective defence, or coming to the aid of an ally under attack.

Japan will not attempt to revise its constitution outright – an option Abe apparently abandoned after accepting he would not win the necessary majorities in parliament and in a nationwide referendum – but will reinterpret the "pacifist" article 9, which prohibits the threat or use of force as a means of settling international disputes.

The change, which is expected to be approved by parliament – where Abe's Liberal Democratic party and its junior coalition partner hold majorities – would allow Japan to exercise collective self-defence for the first time since the end of the second world war.

In practical terms, armed troops could take part in UN peacekeeping operations or be dispatched to "grey zone" emergencies that have not developed into full-blown conflict. Most troubling of all, say critics, the military could come to the aid of an ally, leaving open the possibility that Japan could be dragged into war at Washington's behest.

Abe has made building a better-equipped, more robust military a cornerstone of his second term as prime minister. As a prominent figure among conservative politicians who want to change the country's view of its wartime record, he says the current constitution compromises Japan's ability to defend itself and its allies amid Chinese military aggression and North Korea's nuclear weapons programme.

In a televised address on Tuesday, Abe said Japan would remain a pacifist state and denied that the new policy would mean sending troops into combat zones. Instead, he said it would offer better protection to the Japanese people. The country's navy, for example, would be able to help protect US warships that were fighting to defend Japan, he said.

"This is for the happiness of the Japanese people," Abe said. "Japan's status as a peaceful country will not change."

According to a cabinet document on the change, Japan would only take action if "a country's existence is threatened, and there are clear dangers that the people's right to life, liberty, and the pursuit of happiness would be overturned" by an armed attack on Japan or on "countries with close ties".

But neighbouring countries that were victims of Japanese militarism in the first half of the 20th century have warned Tokyo against abandoning its pacifist principles. "Beijing opposes Japan's act of hyping the China threat," Chinese foreign ministry spokesman Hong Lei told reporters in Beijing. "It's only natural for us to wonder if Japan is going to leave the path of peaceful development that it has long been pursuing."

In Seoul, the foreign ministry spokesman Noh Kwang-il said: "Our position is that the discussions should be held on the basis of the pacifist constitution, dispelling concerns among neighbouring countries stemming from history, and into a direction of contributing to peace and stability in the region."

Abe's push to remove constitutional restraints on the military has also attracted opposition in Japan. An estimated 10,000 people demonstrated outside his official residence on Monday evening, and similar protests were held on Tuesday.

"The current constitution is the result of the sacrifice of more than three million Japanese and more than 20 million Asian victims of war," said Yoshihiko Murata, a 74-year-old protester. "We should value it more." Several recent opinion polls show most voters oppose the lifting of the ban on collective self-defence.

Supporters of the change said Japan is simply adopting the same defence policy as other liberal democracies at a time of increasing uncertainty in the Asia-Pacific. "What we are trying to do now is to play a more proactive role in cooperating with regional countries in setting up a framework to protect the peace and stability of the region," Takeshi Iwaya, who heads the LDP's security research commission, told AP.

Ishida said the effective removal of the last significant constraint on Japan's military had come at a dangerous time for the region, citing Abe's visit to Yasukuni shrine as part of an attempt to foment tension with China and South Korea.

"The state secrets law immediately reminded me of the peace preservation law in 1928, which made it much easier to arrest people and suppress information," Ishida said. "Now we have this new interpretation of article 9 of the constitution that would allow Japanese forces to fight overseas. I am worried that history is repeating itself."